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Insect Resistance?

We keep hearing about resistant strains of bacteria and viruses. It happens because there is always some of the organisms that have a natural ability to survive the action of the medicine. These are not necessarily mutants but the survivors are left to reproduce and most of their progeny contain the now dominant ‘genes’ which allow them to survive and reproduce. Bacteria and virus organisms have a very short life cycle and they quickly develop resistant strains.

Insects can also build up a resistance to a regularly used insecticide by the same natural selection process; the survivors are the only ones left to mate and their progeny have the survival ‘mechanism’. Eventually, the original insecticide doesn’t work at all; the pests keep eating and breeding. To combat this process, another insecticide is usually chosen from a completely different chemical group. Alternating the application using different groups is established good practice.

Natural pyrethrum has no resistance problems. There are a couple of theories: the natural pyrethrum molecule is very complex with its 6 component isomers and variations within those isomers, therefore insects have a hard time combatting and surviving all the actions. Laboratory trials to develop resistance in flies and various grain insect pests have only been nominally successful and this is not replicated in real life situations. The other theory is the choice of Piperonyl butoxide as the synergist to go with natural pyrethrum; (when synthetic pyrethoids used in cotton crops had a resistance problem, PBO was added and insects were again controlled).